1. Field of the Invention
The present invention relates to a method of manufacturing a laminated transformer, and particularly to, for example, a laminated transformer in which coils are formed in a laminate constituted by laminating a plurality of ceramic layers on which electrode layers are formed.
2. Description of the Prior Art
A laminated transformer comprising an alternated laminate consisting of magnetic layers such as ferrite and coil forming electrodes, is known hitherto. As shown in FIG. 7 through FIG. 10, the laminated transformer is manufactured by laminating a plural number of magnetic green sheets 1 (hereinafter, referred to as the magnetic sheets) on which coil electrodes 2, a lead-out electrode 3, through holes 4 and a lead-out electrode 5 are formed. The coil formed by the internal electrodes 2 includes a primary coil 8 and a secondary coil 9. The lead-out electrode 3 at one end of one internal electrode 2 of the primary coil 8 is exposed to the outside. The upper internal electrode 2 is connected to the lower internal electrode 2 by the through hole 4. Meanwhile, the lead-out electrode 5 at one end of the other internal electrode 2 is exposed to the outside at the end surface opposing the lead-out electrodes 3. In such a manner, the primary coil 8 is formed by the internal electrode 2. Similarly, the secondary coil 9 is formed by the internal electrodes 2. Lead-out electrodes 6 and 7 at ends of the internal electrodes 2 are exposed to the outside at opposite surfaces. The locations are adjacent to the lead-out electrodes 3, 5. The alternated laminate of the magnetic sheets 1 and the coil forming internal electrodes 2 is pressed and sintered to form a monolithic chip body. As shown in FIG. 8, external electrodes 10a, 10b, 11a and 11b are formed on the chip body as a laminated transformer. FIG. 9 is a sectional view schematically showing an internal structure of the laminated transformer constituted in such a manner. As such, the primary coil 8 and the secondary coil 9 are embedded in the magnetic body. FIG. 10 is an equivalent circuit diagram of the laminated transformer.
Now, in order to increase the coupling coefficient between the primary and secondary coils in FIG. 9, a distance d between the coils must be reduced. But, in the above-mentioned conventional laminated transformer, as the distance d is reduced, the dielectric strength tends to drop. Particularly, since the dielectric strength of the magnetic body such as ferrite is generally small, the distance d can not be reduced. Also, even as the distance d is reduced in FIG. 9 where the primary and secondary coils are embedded in the magnetic body, the magnetic flux passes between the coil electrodes, and as a result, the coupling coefficient may drop.